Volume control circuit



Feb. 17, 1942. c. J. VANLOON 2273;673

VOLUME QONTROL GIRCUIT F'iieci Feb. ga, 1940 70 A. E NETWORK DE TEL TOR S/GNAL INPUT SOURZE INVENTOR, CAREL J. I/AN LOON 7'0 SIGNAL SOURCE ATTORNEY.

Patented Feb. 17, 1942 2273,673 VOLUME common omoorr Gare] Jan van Loon, Eindhoven, Netherlands, as-

sgnor to Radio Corporation of America, a cox poration of Delaware Application February 28, 1940, Serial N0. 321,180 In the Netherlands July 1, 1939 In thermionic amplifiersthe degree of amplification is mostly controlled by variation of the grid bias of one or more of the tubes, in this case named controlled tubes. This entails the disadvantage that these tubes cannot begiven the characteristic which is most advantageous for its main functon, snce the dsplacement of the workng point along the characteristc curve has to be considered.

According to the present invention this drawback is obviated by making use of beam deflection within one or more of the controlled tubes for the desired control of the degree of amplificaton. This er1ables these tubes to be operated at constant grid bias wth the most advantageous characteristic.

The arrangement is preferably made so that the total electronic beam remains on the average substantially equally strng wth every degree of deflection, in which case the tota1 mean emission current may be caused to determine the grd bias of the tube concerned.

It may then be desirable in some cases that the intensity variations of the deflected part, 1. e. of the undesired part of the beam, brought about by the signal should be applied in anti-phase to the control grid of the controlled tube in question'. This insures a degree of negative feedback varying with the defiecton (and consequently with the signal intensity), which offers various advantages which will be specified hereinafter.

The invention will be more clearly understood by reference to the accompanying drawing, given by way of example, in which Fig. 1 shows one embodirnent of the nvention, and Fig. 2 shows another embodment wth applicaton of variable negative feedback.

Referring to Fig. 1, I is a tube comprising a cathode 2, a control grid 3, a grd 4, a main anode and an auxlary anode 6. To one side of the electronic current is a deflecting plate 1 to which is supplied the control voltage assumed in this case to originate from a known device for automatic volume control (AVC). The sgnal to be' amplified is applied between control grid 3 and ground. The main anode 5 is connected across a tuned circuit 6, and the auxiliary anode 6 is directly connected in a conductive manner to the positive pole of a source of direct current IZ (not shown). Finally the common branch of the two anode circuits comprises a resistance 9 shunted by a smoothing condenser 111.

The device operates as follows:

In the case of weak signals no voltage will be applied to the deflector plate 1, and the total emisson current is divided in a definite manner between the two anodes 5 and 6. The device is preferably to be made so that substantially the whole beam falls on the main anode 5. T0 this end use may be made, for example, of a so-called electronic ptcal system. With increasing intensity of the signal, however, the electronic beam wil1 graduallybe deflected more and more to the right due to the plate 'l becoming more and more negative. The strongest control is naturally obtained when at last almost the whole beam falls on the auxilary anode 6. The sad optical system may also contrbute to this eiect, while a1so the arrangement of two or more deflector plates instead of only one may be useful in this connecton. Of course, the AVC voltage becomes increasingly negative as the carrier strength increases.

Consequently, direct current and signal are going to take more and more their path through the auxiliary anode 6. The anode 6 may be consdered, therefore, as an electron waste electrode, or as an electron diversion electrode. By a correct choice of the constants it can be insured that the signal intensity across the circuit 8 varies only slightly with the intensity of the incoming signal carrier. Furthermore, the total direct current remans constant in the common part of the two anode circuits, i. e. across the resistanoe 9. Thevoltage drop produced across this resistance, and which is equal to the control grid bias, consequently is not vared either. In the case of control it is consequently only the tube characteristic which turns around the cut-oi point, but the working point remains unchanged.

The advantage of this constant (negatve) bias entails, however, a disadvantage which may become troublesome in some cases. In tact, this bias must now be given so high a value that it is still sufficient for the strongest signal that can be expected on the control grid. If the controlled tube is connected, for example, to the aeral circuit and the signal intensty in the aerial is already eifective from the order of magnitude of 1 volt, then signals with a peak Voltage of approximately 10 volts can be expected on the control grid of the controlled tube, and consequently the bias has to be adjusted at least -10 volts, which is objectonable.

T0 obviate this inconvenience, use may be made of the arrangement of Fig. 2. This is quite analogous to that of Fig. 1 with the difi'erence that the condenser [0 in this case is only located in the circuit of the main anode 5, and. a resistance I I which is great as compared with 9 is connected between the dead end of the circuit 8 and the source of direct current supply 12. The total direct current is naturally not effected thereby. The alternating current of the main anode circuit, however, now flows through the condenser 10, and. that of the auxiliary anode largely flows through the resistance 9 to the cathode. The latter point results in negative feedback due to an alternating voltage being produced, in anti-phase with the signal, across the resistance 9 which is now no longer bridged by the condenser |0.

This negative feedback has the twofold advantage that the negative grid-bias and the control factor can now be given a small value. For example, assuming the signal to be attenuated five times at the most by the negative feedback, a bias of 2 volts below the grid voltage at which a grid current just does not flow will be suflicient under the circumstances set out above, and a control factor which is five times smaller, i. e. five times smaller voltage variations on the deflecting plate I, will suflice. It is not desirable to raise the degree of negative feedback, which is controllable with the aid of the resistance 9, much higher, since the negative feedback is effected by the defiected part of the electronic beam which is not used as a useful signal so that the tube noise will increase with the negative feedback. On the other hand, however, the negative feedback varies with the intensity of the defiected signal and consequently with the intensity of the incoming signal, which naturally is desirable.

What is claimed is:

1. In combination with an electron beam tube of the type including an emission element, an output electrode arranged to receive an electron beam from the emission element, a control electrode, means for impressing signals upon the control electrode, an output circuit connected to the output electrode, a signal rectifier, at least one electron diversion electrode disposed within the tube, a beam deflection electrode disposed within the tube, a direct current voltage connection from the beam deflection electrode to the signal rectifier such that the electron beam is defiected from the output electrode to said diversion electrode as the signal amplitude increases, means in the space current circuit includng said emission element and diversion electrode to develop signal voltage, and means for applying the said signal voltage in degenerative phase to said control electrode.

2. In combination with an electron beam tube of the type including an emission element, an output electrode arranged to receive an electron beam from the emission element, a control electrode, means for impressing signals upon the control electrode, an output circuit connected to the output electrode, a signal rectifier coupled to the output circuit, at least one electron waste electrode disposed within the tube, a beam deflection electrode disposed within the tube, a voltage connection from the beam deflection electrode to the signal rectifier such that the electron beam is defiected from the output electrode to said waste electrode as the signal amplitude varies, and means in circuit with the waste electrode for developing signal voltage from diverted. electron flow, and means to apply the signal voltage so developed to the control electrode in degenerative phase.

3. In combination with an electron beam discharge tube of the type comprising a cathode, an output electrode, a waste electrode adjacent said output electrode, a signal control grid surrounding said cathode for modulating the electron discharge from said cathode to said output electrode and a deflecting electrode between said cathode and said output and waste electrodes and. beside the path of electron discharge, a sig nal output circuit connected to said output electrode, a signal rectifier for producing a potential depending on the signal amplitude, a voltage connection between said rectifier and said deflecting electrode for causing said deflecting electrode to pull the electrons from said output electrode to the waste electrode as the signals increase in amplitude, and means responsive to electron flow to the waste electrode for degeneratively feeding signal voltage to the control electrode.

4. In combination with an electron beam tube of the type including an emission element, an output electrode arranged to receive an electron beam from the emission element, means for impressng signals upon the control electrode of the tube, an output circuit connected to the output electrode of the tube, a signal rectifier connected to be responsive in direct current voltage to the signal amplitude, a beam deflection electrode disposed within the tube, a direct current voltage connection from the tube deflection electrode to the signal rectifier such that the electron beam is defiected from the output electrode as the sig nal amplitude increases, and means responsive to electrons defiected from the output electrode for degeneratively applying signal voltage to the control electrode.

5. In combination with a source of signals, an electron discharge tube of the type including an emission element, a pair of positive output electrodes arranged substantially side by side to receive a flow of electrons from said emission element, and a signal input electrode, means for impressing signals from said source upon said input electrode, a signal output circuit connected to one of said output electrodes, an auxiliary electrode disposed within the tube and substantially outside said flow of electrons for controlling the relative flow of electrons from said emission element to each of said output electrodes, means responsive to signal amplitude variatons for varying the voltage of said auxiliary electrode, and additional means responsive to electrons defiected from said one output electrode to the other output electrode for degeneratively applying signal voltage to said input electrode.

CAREL JAN VAN LOON. 

